1. Field of Invention
This invention relates to overload protectors for telecommunications systems.
2. Brief Description of the Related Art
In the telecommunications industry, it is conventional practice to provide overload protectors at central offices for incoming lines. Such protectors protect telephone equipment within customer's premises from damage such as could be caused by overvoltage or overcurrent conditions. These conditions may suddenly occur. For instance, an overvoltage condition may be as a result of a lightning strike to an outside line. Brief overcurrent conditions may cause no damage, especially if an overcurrent condition only slightly exceeds that for which a telecommunications circuit is designed. However, more prolonged overcurrent conditions which are only slightly in excess of the desired maximum value may result in elevated overheating of circuitry to cause gradual burning of insulation and other heat sponsored damage to telephone equipment.
In order to protect a customer's circuitry and equipment from damage caused by overvoltage or overcurrent conditions, an overload protector is provided in each line at the central office. Each protector normally includes two overvoltage protection unit devices and two overcurrent protection devices, the protection devices providing individual overvoltage and overcurrent protection for each tip line and each ring line.
In the main, an overload protector has previously comprised a pair of spaced carbon electrodes or a gaseous discharge arrangement. In overvoltage conditions, the voltage passes across the space between the electrodes or through a gas filled space to ground. On the other hand, an overcurrent protection device normally comprises a coil of wire mounted around a sleeve, the coil being connected in series between a respective outside circuitry terminal and a corresponding central office terminal of the protector. Heat generated in the coil by overcurrent causes solder to melt thereby releasing a pin within the sleeve, the pin then being spring urged into contact with the ground line thereby shorting out the circuit.
In an overload protector of different structure, as described in U.S. Pat. No. 4,434,449, a sleeve holding a coil is mounted upon a line pin and held in a position spaced from a ground line by solder. An overvoltage protection device is mounted upon an end of the sleeve and is spring urged downwardly towards the ground. Melting of the solder causes the sleeve to move towards the ground, thereby shorting out the circuit.
While conventional overload protectors operate in a generally satisfactory manner, they are of complex and expensive structure with the overcurrent and overvoltage devices and ground lines extending substantial distances within housings away from bases of the protectors, with the terminals extending from the bases away from the housings.